For a wireless communication system in which a high frequency spectrum is used, the wireless propagation channel is vulnerable because it may be blocked by various obstacles on its paths. The various obstacles can include a human body, a tree-like thing and/or the like. As compared with the low or medium frequency channel, the high frequency channel would experience more frequent or faster fading and higher obstacle diffraction loss, which may reduce the robustness of the wireless communication system.
In a situation where the wireless propagation channel is totally blocked by such obstacles as a human body or a tree-like thing on its path, the interruption time can be in the level of hundreds of milliseconds, which cannot be solved by HARQ re-transmission which only provides time domain diversity in the level of tens of milliseconds. Thus, the traditional HARQ mechanism which provides time domain diversity is not feasible to the high frequency spectrum. Even though the re-transmission latency of the traditional HARQ mechanism could be extended to the level of hundreds of milliseconds, it would collide with the QoS requirement of latency for e.g., C-MTC traffic, which is undesirable.
In addition, in a wireless communication system where a high frequency spectrum is used, the requirement on reliability sometimes has priority over that on frequency efficiency. An example of such wireless communication system is a millimeter wave based communication system. As previously mentioned, the traditional HARQ mechanism which provides time domain diversity is not feasible to such wireless communication system.